Magnetic (or magnetoresistive) random access memory (MRAM) is a non-volatile memory technology considered to be of great future importance as the standard memory technology for computing devices.
A schematic representation of a typical magnetoresistive memory cell is shown in FIG. 1. A magnetoresistive memory cell (also referred to as a tunneling magneto-resistive or TMR-device) includes a structure having ferromagnetic layers 2, 4 respectively having a resultant magnetic moment vector 5, 6 separated by a non-magnetic layer (tunnel barrier) 3 and arranged into a magnetic tunnel junction (MTJ) 1. Digital information is stored and represented in the magnetic memory cell as directions of magnetic moment vectors in the ferromagnetic layers. More specifically, the resultant magnetic moment vector 6 of one ferromagnetic layer 4 is magnetically fixed or pinned (typically also referred to as the “reference layer”, “pinned layer” or “fixed layer”), while the resultant magnetic moment vector 5 of the other ferromagnetic layer 2 (typically also referred to as the “free layer”) is free to be switched between two preferred directions, i.e., the same and opposite directions with respect to the fixed magnetization 6 of the reference layer 4. The orientations of the magnetic moment vector 5 of the free layer 2 are also known as “parallel” and “antiparallel” states, respectively, wherein a parallel state refers to the same magnetic alignment of the free and reference layers (upper diagram of FIG. 1), while an antiparallel state refers to opposing alignments therebetween (lower diagram of FIG. 1). Accordingly, a logic state of a magnetoresistive memory cell is not maintained by power as in DRAMs, but rather by the direction of the magnetic moment vector of the free layer with respect to the direction of the magnetic moment vector of the reference layer (for instance, a logic “0” in the case of a parallel alignment of magnetic moment vectors and a logic “1” in the case of an antiparallel alignment therebetween).
Depending upon the magnetic states of the free layer, the magnetic memory cell exhibits two different resistance values in response to a voltage applied across the magnetic tunnel junction barrier, wherein the resistance is “low” when the magnetization is parallel and “high” when the magnetization is antiparallel, so that a detection of changes in resistance allows an MRAM-device to provide logic information stored in the magnetic memory element.
A magnetic memory cell typically is written to through the application of magnetic fields from bi- or uni-directional currents. For writing of magnetic memory cells different writing (switching) scenarios are known depending on the actual configuration of the magnetoresistive memory cell such as Stoner-Wohlfahrt-switching or adiabatic rotational switching (toggle-switching) which are well-known to those skilled in the art and therefore need not be further detailed here.
To be useful in present day electronic devices, such as digital cameras or the like, very high density arrays of magnetic memory cells must be used, thus rendering a scaling-down of MRAM cells one of the most important issues, which, however, requires several problems to be solved.
Down-scaling of MRAM cells requires smaller and smaller magnetic tunnel junctions, which proves problematic, since for a given aspect ratio and free layer thickness, the activation energy, being dependent on the free layer volume, scales down like w, where w is the width of the magnetic cell. Otherwise, in down-scaling, the switching fields increase roughly like 1/√{square root over (w)}, so that magnetic field selected switching becomes ever harder, but at the same time the magnetic cells loose their information more and more rapidly due to thermal activation. A major problem with having a small activation energy (energy barrier) is that it becomes extremely difficult to selectively switch one MRAM cell in an array, where selectability is seen to allow switching without inadvertently switching other MRAM cells. The memory cells therefore still need to retain a sizeable shape or induced anisotropy in order to maintain thermal stability.
Reference is now made to FIG. 2 showing a diagram in which the energy barrier height ΔE for switching of magnetic moment vector 5 of magnetic free layer 2 of rectangular MTJ 1 of FIG. 1 having lateral dimensions L for length and 1 for width (see insert) and a low thickness of about 2 nm is plotted against its width l. It is further assumed that magnetization of the magnetic free layer 2 is aligned along directions ±x. Considering a simple Arrhenius law with a 0.1 nsec characteristic attempt time, requesting a ten years stability is equivalent to setting the barrier height between stable states (−x and −x) at about 45 kBT (T=300° K., room temperature, kB is Boltzmann constant).
As can be seen from FIG. 2, an aspect ratio L/1=2 proves sufficient for overcoming the energy barrier height lower limit criterion if 1 remains greater than about 60 nm. A slight increase of the aspect ratio pushes the limit further out. It also becomes clear that as sizes shrink down, the superparamagnetic limit becomes closer and closer.
Another problem in scaling down magnetoresistive memory cells may be seen in that in the case of magnetic field selected switching of memory cells the cell sizes need to be smaller than sizes of the current lines for generating of magnetic fields in order to ensure essentially homogeneous magnetic fields over the whole memory cell area.
In an attempt to overcome the above problems, a new concept of writing to magnetoresistive memory cells has been recently proposed, where the reversal of the magnetic moment vector of the magnetic free layer is generated not by external magnetic fields but by spin-polarized electrons passing perpendicularly through the stack of memory cell layers. For a detailed description of that concept, see for instance seminal U.S. Pat. No. 5,695,864 to Slonczewski and U.S. Pat. No. 6,532,164 to Redon et al., the disclosures of which are incorporated herein by reference.
In the above new concept, by sending an electric current through a magnetic layer having a particular magnetization, spins of electrons are oriented by quantum-mechanical magnetic exchange interaction with the result that the current electrons leave the magnetic layer with a polarized spin. Alternatively, where spin-polarized electrons are passed through a magnetic layer having a particular magnetic moment vector in a preferred easy axis direction, these spin-polarized electrons will cause a continuous rotation of the magnetic moment vector which may result in a reversal of the magnetic moment vector along its easy axis. Hence, switching of the magnetic moment vector between its two preferred directions along the easy axis may be effected by passing spin-polarized electrons perpendicularly through the magnetic layer.
Recent experimental data (see S. I. Kiselev et al., Nature 425 (2003), 380 and W. H. Rippard et al., Phys. Rev. Lett. 92 (2004) 027201) confirm the very essence of magnetic moment transfer as a source of magnetic excitations and, subsequently, switching. These experiments confirm theoretical predictions (see J. C. Sloncezwski, J. Magn. Magn. Mater. 159 (1996) Ll and M. D. Stiles & A. Zangwill, Phys. Rev. B66, (2002) 014407) stating that the leading torque term acting on the magnetization under conditions of spin-polarized DC current is proportional to:
            ⅆ      m              ⅆ      t        ∝      P    ⁡          [              m        ×                  (                      m            ×            p                    )                    ]      where m, p and P are the magnetization direction in space, the polarization direction of the electron current (density per unit area J) and a polarization function, respectively. A direct inspection of above equation indicates that the torque will be maximum when p is orthogonal to m.
Reference is now made to FIGS. 3A and 3B, where a schematic representation of both a magnetic free layer 2 and a magnetic layer 7 for spin-polarizing of current electrons in a stacked arrangement is shown. In that configuration, the magnetic free layer 2 is provided with a magnetization easy axis where a magnetic moment vector 5 is free to be switched between two preferred directions thereof. Magnetic layer 7 is provided with a fixed magnetic moment vector 8 being perpendicular to the magnetic moment vector 5 in the configuration of FIGS. 3A and 3B. FIG. 3A illustrates a case where a current density J of an electron current (not illustrated) flowing perpendicularly through the layers is assumed to be nil, while in FIG. 3B the current density J is assumed to be different from zero. Accordingly, on the one hand, in FIG. 3A where no current is passing through the layers, magnetic moment vector 5 remains unchanged, while, on the other hand, in FIG. 3B, electrons passing through the layers are spin-polarized when flowing through magnetic layer 7 by the effect of magnetic exchange interaction. If a polarization direction p of the current electrons belongs to the plane of the magnetic free layer 2, then rotation of the magnetic moment vector 5 occurs in the plane of magnetic free layer 2 and the torque becomes nil when m becomes parallel to p (that case is not shown in FIGS. 1A, 1B). Alternatively, if p is perpendicular to the plane of the magnetic free layer 2 (case shown in FIGS. 1A, 1B), then the initial torque pulls the magnetic moment vector 5 out of its plane, thus creating a demagnetizing field HD perpendicular to the magnetic free layer 2 plane, with the result that a precession movement of the magnetic moment vector 5 around the demagnetizing field HD may now take place.
In other words, in a magnetic element such as the soft element of an MRAM cell, the magnetization direction though not far from being uniform fails to be so as a result of demagnetizing effects. Coherence during magnetic switching may nevertheless be preserved if the field exerting a torque on the magnetization is perpendicular to the soft layer. In order to achieve this, the best strategy is to apply a magnetic field normal to the mean magnetization direction within the soft element and in the plane of the layer. The initial torque γ0[m×Ha], where γ0, m, Ha are a gyromagnetic ratio, magnetization vector and applied magnetic field, respectively, pulls the magnetization out of the plane leading to the growth of a demagnetizing field that remains essentially normal to the plane of the layer. The magnetization may now precess around the demagnetizing field under the torque γ0[m×HD], where HD is the demagnetizing field.
In order to observe precessional switching, three conditions have to be fulfilled, namely, both the rise and fall times of the field pulse need to be “short” and the length of the pulse has to be tailored very accurately, where “short” means a time small when compared to time requested for the magnetization to make half a turn. Let T and f be the period and precession frequency, respectively. A half a turn rotation means a time equal to T/2. One has T=1/f and f depends on the amplitude of the demagnetizing field: ω=2Πf=γ0Hd. On the other hand, the demagnetizing field scales with the angle of the magnetization out of the sample plane.
An example may illustrate this: suppose the magnetization leaves its plane by an angle of ∂=10°, then the demagnetizing field amplitude will amount to about Hd≈Ms sin (10°). For a typical soft material with saturation induction μ0Ms=1 Tesla, this means a precession frequency equal to f=(ω/2Π)=γ0Ms sin(10°)≈5 GHz. The period then amounts to 200 picoseconds, and the time necessary for a half turn rotation would typically be T/2=100×10−12 sec (100 picoseconds (ps)). In summary, owing to values chosen in the sample, the pulse length should be close to 100 ps and the fall and rise times much shorter than 100 ps. Laboratory realizations allow for pulse rise and fall times of the order of 20 ps.
Precessional switching is a very robust and fundamental effect. In a large scale memory, however, due to various sources of impedance, it is expected that maintaining such an accuracy in the definition of the field pulses might prove extremely problematic.
In order to result in a desired reversal of the free magnetic moment vector, precession movement has to be controlled appropriately, which, however, has not been demonstrated in prior art.